1. Field of the Invention
This invention concerns an apparatus and method utilizing gamma radiation monitoring techniques for determining the relative mass fractions of steam and water passing through a conduit.
2. Description of the Prior Art
There are numerous situations in which it is desirable to characterize a substance by the concentration of water or some other hydrogenous material contained therein. In one class of these situations the density of the hydrogenous material is macroscopically constant across the substance and a sample of the substance may be readily isolated for analysis. Substances generally falling within this class include grain, wood, soils, slurries, etc. Where time for analysis is not limited, a sufficiently accurate measure of the concentration of hydrogenous material in such substances can generally be obtained through the use of techniques well familiar to those skilled in the art.
However, where the substance in question is at an extremely high temperature, is highly corrosive, exists only transiently or is not readily accessible, obtaining an accurate measure of the concentration of hydrogenous material therein can be difficult or impossible by traditional methods of chemical or physical analysis. One class of substances that in certain situations has proven especially resistant to common methods of analysis includes vapor-liquid mixtures such as wet steam.
It is frequently important that the steam quality at some point in a process or apparatus utilizing steam be known. Steam quality is defined as the mass ratio of the pure steam vapor to the total fluid within a steam sample and is usually expressed as a percentage. Commonly utilized methods of measuring steam quality include sodium titration analysis, calorimetric analysis and relative volume analysis by phase separation. These methods require fluid access to the steam and are time consuming and labor-intensive. Accordingly, it has proven impracticable to provide continuous, rapid steam quality monitoring at any point in a steam system for which access to the steam itself cannot be obtained. Without such continuous rapid analysis, accurate control of steam quality often cannot be attained. Furthermore, existing methods of chemical and physical steam quality analysis are especially inadequate in certain applications due to the increasing inaccuracy of such methods with increasing steam pressure and temperature.
It would be especially desirable to develop a simple and accurate steam quality monitoring system for use in conjunction with steam injection systems for oil fields. It is becoming an increasingly common practice to inject high pressure, high quality steam into an oil-bearing formation to stimulate the production of oil therefrom. The injected steam serves to heat the hydrocarbons in the formation causing their viscosity to drop and, hence, their rate of flow through the formation to increase. The steam also provides a displacing action. In order to optimize the efficiency of this recovery technique and to improve reservoir performance predictions, it is important that the steam quality of the injected fluid be known at the point of injection into the well formation. The steam quality is preferably maintained at an optimum value dependent on the specific nature of the operation. It is desirable that an apparatus and method be provided for constantly monitoring the steam quality at the point of injection of the steam water mixture into the formations being flooded and, in response to deviations between the desired and the actual steam quality, to automatically adjust the operation of the steam generator to maintain the steam quality at the desired value. It is further desirable that such steam quality monitoring apparatus be so constructed as to be suitable, with a minimum of adjustment, for use within any steam injection well within a wide range of configurations. It is also desirable that the apparatus and method provide results accurate at low steam pressures as well as above 11.7 MPa (1700 psi), a point beyond which most traditional methods of steam quality monitoring become either impracticable or inaccurate. It is further desirable that the apparatus and method be accurate over a wide range of steam quality extending above 70%, where the total amount of hydrogenous material within the conduit is relatively small.
It is important to note that as the steam-water mixture travels downhole the steam may condense when flowing into a well recently placed on stream, until the casing material is heated sufficiently. Alternatively, the steam-water mixture may be geothermally heated sufficiently in passing downward in the well to cause an increase in the steam quality prior to injection into the formation. It is imperative, therefore, that a steam quality measurement device be developed that may be lowered into the well to a position above the location where the steam-water mixture enters the formation.
Two different apparatus that may be used for measurement of a hydrogenous material are disclosed in U.S. Pat. Nos. 4,499,380 and 4,057,729.
In U.S. Pat. No. '380 issued Feb. 12, 1985, an apparatus is disclosed that measures steam quality and is mounted at a surface location outside of the steam carrying conduit. The apparatus includes a source of fast neutrons, a moderating medium, and a neutron detection system positioned adjacent the steam-water mixture which provides an output signal representative of the time rate of occurrence of neutron detection.
The disadvantages of this system is its overall bulkiness and its inability to measure the steam quality of the steam-water mixture at the point in the well where the mixture enters the formation. The U.S. Pat. No. '380 apparatus can only determine the steam quality at the surface of the well.
A system similar to the U.S. Pat. No. '380 for use in determining the concentration of a cellulose slurry has also been disclosed in U.S. Pat. No. '729, issued Nov. 8, 1977. This system includes a source of fast neutrons suspended within the slurry and a relatively bulky thermal neutron detection system positioned within the slurry at a spaced distance from the source. It should be noted that a fast neutron moderation/attenuation shield system must be included in these devices for them to operate properly. From the detection rate of thermalized neutrons, the concentration of the water within the slurry can be determined. Drawbacks to such a system are that no provision is made for monitoring samples of low hydrogen density such as that found in normal steam quality ranges and that in many circumstances it is impracticable to position either a source or a detector within a sample.
For example, the overall intrusive bulkiness of a system of this nature would inherently cause steam quality measurement errors if inserted into a flowing steam injection well. The pressure disturbances caused by such a device if it is placed in a typically pressure sensitive steam-water mixture would tend to yield questionable results at best.
An apparatus and method need be developed therefore that measures the steam quality of a flowing steam-water mixture immediately before the mixture flows into a subterranean formation. The apparatus therefore needs to be positionable downhole in a steam injection well, and be sufficiently compact and non-intrusive so as to yield reliable steam quality measurements.